1. Field of the Invention
The present invention generally relates to an energy storage device, and more particularly to a bipolar double layer capacitor-type energy storage device, and to methods for manufacturing the same.
2. Description of the Related Art
Energy Storage Devices
There has been significant research over the years, relating to useful reliable electrical storage devices, such as a capacitor or a battery. Large energy storage capabilities are common for batteries; however, batteries also display low power densities. In contrast, electrolytic capacitors possess very high power densities and a limited energy density. Further, carbon based electrode double-layer capacitors have a large energy density; but, due to their high equivalent series resistance (ESR), have low power capabilities. It would therefore be highly desirable to have an electrical storage device that has both a high energy density and a high power density.
A recent review by B. E. Conway in J. Electrochem. Soc., vol. 138 (#6), p. 1539 (June 1991) discusses the transition from xe2x80x9csupercapacitorxe2x80x9d to xe2x80x9cbatteryxe2x80x9d in electrochemical energy storage, and identifies performance characteristics of various capacitor devices.
D. Craig, Canadian Patent No. 1,196,683, in November 1985, discusses the usefulness of electric storage devices based on ceramic-oxide coated electrodes and pseudo-capacitance. However, attempts to utilize this disclosure have resulted in capacitors which have inconsistent electrical properties and which are often unreliable. These devices cannot be charged to 1.0 V per cell, and have large, unsatisfactory leakage currents. Furthermore, these devices have a very low cycle life. In addition, the disclosed packaging is inefficient.
M. Matroka and R. Hackbart, U.S. Pat. No. 5,121,288, discusses a capacitive power supply based on the Craig patent which is not enabling for the present invention. A capacitor configuration as a power supply for a radiotelephone is taught; however, no enabling disclosure for the capacitor is taught.
J. Kalenowsky, U.S. Pat. No. 5,063,340, discusses a capacitive power supply having a charge equalization circuit. This circuit allows a multicell capacitor to be charged without overcharging the individual cells. The present invention does not require a charge equalization circuit to fully charge a multicell stack configuration without overcharging an intermediate cell.
H. Lee, et al. in IEEE Transactions on Magnetics, Vol. 25 (#1), p.324 (January 1989), and G. Bullard, et al., in IEEE Transactions on Magnetics, Vol. 25 (#1) p. 102 (January 1989) discuss the pulse power characteristics of high-energy ceramic-oxide based double-layer capacitors. In this reference various performance characteristics are discussed, with no enabling discussion of the construction methodology. The present invention provides a more reliable device with more efficient packaging.
Carbon electrode based double-layer capacitors have been extensively developed based on the original work of Rightmire, U.S. Pat. No. 3,288,641. A. Yoshida et al., in IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-10, #1, P-100-103 (March 1987) discusses an electric double-layer capacitor composed of activated carbon fiber electrodes and a nonaqueous electrolyte. In addition, the packaging of this double-layer capacitor is revealed. This device is on the order of 0.4-1 cc in volume with an energy storage capability of around 1-10 J/cc.
T. Suzuki, et al., in NEC Research and Development, No. 82, pp. 118-123, July 1986, discloses improved self-discharge characteristics of the carbon electric double-layer capacitor with the use of porous separator materials on the order of 0.004 inches thick. An inherent problem of carbon based electrodes is the low conductivity of the material resulting in a low current density being delivered from these devices. A second difficulty is that the construction of multicell stacks is not done in a true bipolar electrode configuration. These difficulties result in inefficient packaging and lower energy and power density values.
Additional references of interest include, for example:
The state of solid state micro power sources is reviewed by S. Sekido in Solid State Ionics, vol. 9, 10, pp. 777-782 (1983).
M. Pham-Thi et al. in the Journal of Materials Science Letters, vol. 5, p. 415 (1986) discusses the percolation threshold and interface optimization in carbon based solid electrolyte double-layer capacitors.
Various disclosures discuss the fabrication of oxide coated electrodes and the application of these electrodes in the chlor-alkali industry for the electrochemical generation of chlorine. See for example: V. Hock, et al. U.S. Pat. No. 5,055,169 issued October 8, 1991; H. Beer U.S. Pat. No. 4,052,271 issued Oct. 4, 1977; and A. Martinsons, et al. U.S. Pat. No. 3,562,008 issued Feb. 9, 1971. These electrodes, however, in general do not have the high surface areas required for an efficient double-layer capacitor electrode.
It would be useful to have a reliable long-term electrical storage device, and improved methods to produce the same. It would also be desirable to have an improved energy storage device with energy densities of at least 20-90 J/cc.
Packaging of Energy Storage Devices
As mentioned above, there has been significant research over the years regarding electrical storage devices of high energy and power density. The efficient packaging of the active materials, with minimum wasted volume, is important in reaching these goals. The space separating two electrodes in a capacitor or a battery is necessary to electrically insulate the two electrodes. However, for efficient packaging, this space or gap should be a minimum. It would therefore be highly desirable to have a method to create a space separator or gap that is substantially uniform and of small dimension (less than 5 mil (0.0127 cm).
A common way to maintain separation between electrodes in an electrical storage device with an electrolyte present (such as a battery or capacitor) is by use of an ion permeable electrically insulating porous membrane. This membrane is commonly placed between the electrodes and maintains the required space separation between the two electrodes. Porous separator material, such as paper or glass, is useful for this application and is used in aluminum electrolytic and double layer capacitors. However, for dimensions below 1 or 2 mil (0.00254 to 0.00508 cm) in separation, material handling is difficult and material strength of the capacitor is usually very low. In addition, the open cross-sectional areas typical of these porous membrane separators are on the order of 50-70%.
Polymeric ion permeable porous separators have been used in carbon double layer capacitors as discussed by Sanada et al. in IEEE, pp.224-230, 1982, and by Suzuki et al. in NEC Research and Development, No. 82, pp. 118-123, July 1986. These type of separators suffer from the problem of a small open area which leads to increased electrical resistance.
A method of using photoresist to fill voids of an electrically insulating layer to prevent electrical contact between two electrode layers for use as a solar cell is disclosed by J. Wilfried in U.S. Pat. No. 4,774,193, issued Sep. 27, 1988.
A process of creating an electrolytic capacitor with a thin spacer using a photosensitive polymer resin solution is disclosed by Maruyama et al in U.S. Pat. No. 4,764,181 issued Aug. 16, 1988. The use of solution application methods described with a porous double-layer capacitor electrode would result in the undesirable filling of the porous electrode.
Additional references of general interest include U.S. Pat. Nos. 3,718,551; 4,816,356; 4,052,271; 5,055,169; 5,062,025; 5,085,955; 5,141,828; and 5,268,006.
All of the applications, patents, articles, references, standards, etc. cited in this application are incorporated herein by reference in their entirety.
In view of the above, it would be very useful to have one or more methods to produce a reliable small space separation between electrodes in electrical storage devices with a large open cross-sectional area. The present provides these methods.
The present invention relates to a novel electrical storage device that has both a high energy density and a high power density.
It is an object of the present invention to provide new methods for manufacturing the storage device.
It is also another object of the present invention to provide a reliable long-term electrical storage device, and improved methods to produce the same.
It is a further object of the present invention to provide efficient packaging of an electrical storage device by reducing the gap between the anode and cathode, which reduces the electrical resistance of the ionically conducting electrolyte.
Briefly, the foregoing and other objects are attained by an energy storage device such as a capacitor, which includes a plurality of cells in a bipolar configuration. The cells are stacked and bonded together, to impart to the device an integral and unitary construction.
Each cell includes two electrically conductive electrodes that are spaced apart by a predetermined distance. The cell also includes at least one dielectric gasket that is interposed, on the perimeter in relation to each other, between the electrodes, for separating and electrically insulating these electrodes.
When the electrodes, and the gaskets are bonded together, at least one fill gap is formed for each cell. Each cell also includes a high surface area (porous) electrically conductive coating layer that is formed on one (or, more) surface of each electrode. This coating layer optionally includes a set of closely spaced-apart peripheral microprotrusions, and a set of distally spaced-apart central microprotrusions. These microprotrusions are formed by novel screen printing or photolithographic printing methods. These microprotrusions impart structural support to the cells, and provide additional insulation between the electrodes.
An ionically conductive medium fills the cell gap and pores of the high surface area coating.
The present invention also discloses materials and the processes to edge seal the electrodes which are used in the manufacture of high electrical energy and power density devices, such as capacitors.